Method for the production of cyclohexanol from benzole

ABSTRACT

A process for preparing cyclohexanol from benzene by a) preparing cyclohexene by hydrogenating benzene in the presence of a catalyst and b) preparing cyclohexanol by hydrating the cyclohexene in the presence of a catalyst, comprises carrying out steps a) and b) in a reaction facility which has a bottom region at the lower end, a top region at the upper end and a reaction zone between the top region and the bottom region which contains the catalyst according to steps a) and b), evaporating a portion of the benzene using the heat of reaction in the reaction zone, condensing it in the top region and returning it to the reaction zone, and withdrawing a reaction mixture containing cyclohexanol in the bottom region.

The present invention relates to a process for preparing cyclohexanolfrom benzene by

-   a) preparing cyclohexene by hydrogenating benzene in the presence of    a catalyst and-   b) preparing cyclohexanol by hydrating the cyclohexene in the    presence of a catalyst,    -   which comprises    -   carrying out steps a) and b) in a reaction facility which has    -   a bottom reaction at the lower end,    -   a top reaction at the upper end and    -   a reaction zone between the top reaction and the bottom reaction        which contains the catalyst according to steps a) and b),    -   evaporating a portion of the benzene using the heat of reaction        in the reaction zone, condensing it in the top reaction and        returning it to the reaction zone, and    -   withdrawing a reaction mixture containing cyclohexanol in the        bottom reaction.

According to Ullmann's Encyclopedia of Industrial Chemistry, 5thEdition, Volume A8, VCH Verlagsgesellschaft mbH, Weinheim, Germany,1987, page 222, 6^(th) paragraph, cyclohexanol is an importantintermediate in preparing adipic acid and caprolactam which are bothmonomers for preparing important polymers, such as nylon 6 and 66. Inaddition, cyclohexanol finds use as a solvent and cleaning agent and inpreparing plasticizers, insecticides and fragrances.

Various continuous processes are known for preparing cyclohexanol.

According to Ullmann's Encyclopedia of Industrial Chemistry, loc cit.,p. 221, paragraph 2.4, one of these comprises the hydrogenation ofbenzene to cyclohexene in the presence of a catalyst and the hydrationof the cyclohexene in the presence of a catalyst to cyclohexanol.

As is well known, there are two main problems in this process: theundesirably high extent of cyclohexane formation from the hydrogenationof benzene to cyclohexane and the subsequent difficult removal ofcyclohexene from the benzene/cyclohexane/cyclohexene mixture.

In order to suppress overhydrogenation of benzene to cyclohexane, thehydrogenation may be carried out using only partial benzene conversion.However, this has the disadvantage that relatively large apparatus isrequired for the hydrogenation and that in addition it is necessary toremove unconverted benzene, as well as the cyclohexane which occursdespite the only partial conversion of benzene, from the productmixture. Since benzene and cyclohexane form an azeotrope, but onlybenzene can be hydrogenated to cyclohexene, the separation of thereaction products in this process variant also presents considerableproblems.

It is an object of the present invention to provide a process whichallows the preparation of cyclohexanol from benzene in a technicallysimple and economical manner while avoiding the abovementioneddisadvantages.

We have found that this object is achieved by a process as defined atthe outset.

According to the invention, the preparation of cyclohexanol from benzeneis carried out in a reaction facility which has

-   a bottom reaction at the lower end,-   a top reaction at the upper end and-   a reaction zone between the top reaction and the bottom reaction    which contains the catalyst according to steps a) and b).

Preferred reaction facilitys include distillation columns as described,for example, in Kirk-Othmer, Encyclopedia of Chemical Technology, 3^(rd)Ed., Vol. 7, John Wiley & Sons, New York, 1979, pages 870-881, such astray columns, for example sieve tray columns or bubble cap tray columns,columns with structured or random packings. Particular preference isgiven to columns with structured packings.

The reaction zone may be disposed within the distillation column.

The reaction zone may be disposed outside the distillation column. Inthis case, the pressure in the reaction zone and the pressure in thedistillation column may be identical or different.

In a preferred embodiment, the reaction facility may contain a furtherzone (“separating zone”) between the bottom reaction and reaction zonewhich is free of catalysts from step a) and step b) and has atheoretical number of plates in the range from 5 to 50, preferably from20 to 40, in particular from 15 to 25.

An advantageous further zone is the separating section of a distillationcolumn, which is that section of a distillation column that is disposedbetween the bottom section and the top section of a customarydistillation column, as described in, for example: Kirk-Othmer,Encyclopedia of Chemical Technology, 3^(rd) Ed., Vol. 7, John Wiley &Sons, New York, 1979, pages 870-881, such as tray columns, for examplesieve tray columns or bubble cap tray columns, columns with structuredor random packings.

In a particularly preferred embodiment, the reaction facility is apressure column.

The process according to the invention can advantageously be carried outat a pressure in the range from 0.1 to 3.5 MPa, preferably from 0.5 to2.5 MPa.

This results in temperatures in the reaction zone in the range from 70to 220° C., preferably from 120 to 190° C.

In a further preferred embodiment, the reaction facility may have ameans of withdrawing gases at the upper end of the top section.

According to the invention, the reaction zone contains catalystssuitable for carrying out the reactions in steps a) and b). Inprinciple, useful catalysts include those known per se for steps a) andb).

In a preferred embodiment, catalysts can be used in step a) which use atleast one element of group VIII, preferably ruthenium, as thecatalytically active component or mixtures of these elements can be usedas catalyst. If desired, such catalysts may contain at least one furthertransition metal of a group other than VIII, preferably zinc, ormixtures of such metals, as a dopant.

Such catalysts are described, for example, in EP-A-1048349.

In a further preferred embodiment, a catalyst can be used in step b)which comprises at least one zeolite, such as ZSM-5, ZSM-11, ZBM-10,MCM-22, MCM-36, MCm-49 or PSH-3, or or a mixture thereof.

Such catalysts are described, for example, in DE-A-19951280.

In a particularly preferred embodiment, mixtures of the catalystsmentioned for step a) and step b) may be used.

In a very particularly preferred embodiment, a bifunctional catalyst maybe used which simultaneously catalyzes steps a) and b).

The quantities of catalyst for steps a) and b) may be easily determinedusing the catalyst space velocities known for these catalysts for eachreaction and the conversions chosen in the process according to theinvention, and the catalyst quantities may be easily optimized by a fewsimple preliminary experiments.

Benzene is advantageously added to the top region of the reactionfacility, in particular in the liquid phase.

Water is also advantageously added to the top region of the reactionfacility, in particular in the liquid phase.

In particular, benzene and water may be added as a liquid phase mixtureto the top reaction of the reaction facility.

The mixing ratio of benzene to water is determined by the stoichiometryof the reaction and also by the water quantity withdrawn in the bottomreaction. A particularly useful mixing ratio of benzene to water is inthe range from 1:2 to 5:1 mol/mol, preferably from 1:1 to 3:1 mol/mol.

The hydrogen required for the reaction may advantageously be added tothe reaction facility below the reaction facility. When the reactionzone contains a separating zone, preference is given to adding thehydrogen to the reaction facility between the separating zone and thereaction zone.

The mixing ratio of benzene to hydrogen is determined by thestoichiometry of the reaction and also by the quantity of cyclohexanewithdrawn in the bottom reaction.

The reactions taking place in the reaction zone are exothermic overall,i.e. generate heat. According to the invention, the reaction is carriedout by using the heat of reaction to evaporate a portion of the benzene,condensing it in the top reaction and returning it to the reaction zone.

According to the invention, a reaction mixture is withdrawn in thebottom reaction which contains cyclohexanol.

In an advantageous embodiment, the reaction mixture contains less than10% by weight, preferably less than 1% by weight, of benzene, and inparticular the reaction mixture is free of benzene.

The selectivity for cyclohexanol in steps a) and b), based on benzene,and the benzene content of the reaction mixture withdrawn in the bottomreaction may advantageously be controlled by means of the energy inputinto the reaction facility, in particular in the bottom reaction.

Advantageously, the conversion is controlled in such a manner that thereaction mixture, based on the total weight of the reaction mixture,consists of

-   from 20 to 40% by weight, in particular from 25 to 30% by weight, of    cyclohexanol,-   from 40 to 70% by weight, in particular from 50 to 70% by weight, of    cyclohexane,-   from 1 to 10% by weight, in particular from 2 to 10% by weight, of    water,-   and any remainder of benzene, although preferably none.

The achievement of the underlying object of the present invention by theprocess according to the invention is surprising, since, as is wellknown, the hydrogenation of cyclohexene to cyclohexane is rapid, but thehydration of cyclohexene to cyclohexanol is slow and, in addition, saidhydration is an equilibrium reaction which lies on the side ofcyclohexene.

EXAMPLE 1

A pressure column of 70 mm diameter having a bottom region, a separatingzone having 20 bubble cap trays, a reaction zone which contains 750 cm³of a catalyst comprising 0.1% by weight of ruthenium and 0.1% by weightof zinc on ZBM-10 extrudates (1.5 mm diameter) in Montz Katapak, and atop region was charged with a liquid mixture of 500 g/h of benzene and38 g/h of water at 95° C. in the top region and with 34 g/h of hydrogenbetween the reaction zone and the separating zone at a column pressureof 1.216 MPa (12 bar) measured in the bottom region. The temperature inthe bottom region was 180° C., the energy input in the bottom region 0.6kW.

The reaction mixture withdrawn in the bottom reaction resulting from thebenzene, water and hydrogen quantities added had the followingcomposition:

-   29% by weight of cyclohexanol-   61% by weight of cyclohexane-   10% by weight of water-   no benzene.

1. A process for preparing cyclohexanol from benzene by a) preparingcyclohexene by hydrogenating benzene in the presence of a catalyst andb) preparing cyclohexanol by hydrating the cyclohexene in the presenceof a catalyst, which comprises carrying out steps a) and b) in areaction facility which has a bottom region at the lower end, a topregion at the upper end and a reaction zone between the top region andthe bottom region which contains the catalyst according to steps a) andb), evaporating a portion of the benzene using the heat of reaction inthe reaction zone, condensing it in the top region and returning it tothe reaction zone, and withdrawing a reaction mixture containingcyclohexanol in the bottom region.
 2. A process as claimed in claim 1,wherein the benzene is introduced into the top region.
 3. A process asclaimed in claim 1, wherein the selectivity for cyclohexanol from stepsa) and b), based on benzene, is controlled by means of the energy inputinto the bottom region.
 4. A process as claimed in claim 1, wherein thereaction mixture withdrawn from the bottom region is free of benzene andcyclohexene.
 5. A process as claimed in claim 1, wherein a further zoneis disposed between the bottom region and the reaction zone which isfree of the catalysts from steps a) and b) and has a theoretical numberof plates in the range from 5 to
 50. 6. A process as claimed in claim 1,wherein the process is carried out at a pressure in the range from 0.1to 3.5 MPa, measured in the bottom region of the reaction facility.
 7. Aprocess as claimed in claim 1, wherein the reaction facility used is adistillation column.